Hi blackhawk and welcome .
Your problems are all connected with low maintenance and water quality and are easily soluble with just raising WCs and maintenance .
First off , get rid of those Yoyos ASAP because discus won't take their attacks much longer before they get week and sick .
Secondly , MTS heavy infestation is a sure sign that your bottom has too much leftover poop or food that needs to be siphoned daily . 40 PPM nitrates is another sign that you need to raise your WCs and maintenance and asking for trouble when it comes to discus health .
Either siphon out the sand and keep a BB tank or keep it frequently siphoned and clean of poop and debris .
Discus scratching is also related to sub par water quality.
Algae are also related with a nutrional disbalance and excess nitrates and phosphates in water column , so they should also die off once you start to keep the water quality on top and spotless .
They are also dependant of light , so lowering the light intensity and Photoperiod should weaken them too .
And finnaly your tank size is too small for that bioload and you should consider a bigger tank in near future if you want to keep a healthy batch of 6 discus in it .
Good luck and please send some photo and info about your tank and equipment .